1. Field of Invention
The present invention relates to a method of manufacturing a crystal of silicon through use of the Czochralski method (CZ method).
2. Description of Related Art
When a crystal of silicon is pulled from silicon melt within a quartz crucible in accordance with the CZ method, the inner surface of the crucible is exposed to the silicon melt, which is maintained at a high temperature. As a result, the condition of the inner surface of the crucible changes and deteriorates. In particular, the vitreous silica changes to a crystalline form, .beta.-cristobalite. The phase change occurs at many nucleation sites on the inner surface of the crucible. As dissolution of the inner layer occurs, there is potential of undercutting of the .beta.-cristobalite formations. If the undercutting is severe enough, the formation may be released into the silicon melt. If the .beta.-cristobalite separates from the inner surface of the crucible and adheres to the surface of a crystal of silicon being pulled, a dislocation may be generated in the crystal.
Especially in manufacture of a crystal of silicon having a diameter equal to or greater than 8 inches, needed to cope with the recent increases in the degree of integration and the degree of accuracy of semiconductor devices, the inner surface of a quartz crucible is exposed to longer operation times and higher temperatures. This leads to increased crucible deterioration and therefore the problem of dislocation generation is apt to occur. In order to manufacture large diameter crystals, much larger crucibles must be used in order to minimize manufacturing costs. For example, in order to manufacture a crystal of silicon having a diameter equal to or greater than 8 inches, a quartz crucible having a large diameter equal to or greater than about 457 mm must be used. In order to melt a large amount of silicon material within such a large-diameter quartz crucible and to maintain the thus-obtained silicon melt, a graphite heater, generally used as a heating element, must be heated to higher temperatures. As a result, the quartz crucible itself is heated to an even higher temperature. The higher the temperature to which the quartz crucible is heated, the higher the possibility of occurrence of deterioration of the inner surface of the crucible that is in contact with the high-temperature silicon melt.
The above-mentioned deterioration of the inner surface of a quartz crucible is generally known to occur in many of the various known CZ pulling methods. For example, deterioration occurs in the multi-pulling method, wherein a silicon material is recharged into the crucible so as to manufacture a plurality of crystals of silicon from the same crucible. See Semiconductor Silicon Crystal Technology, Fumio Shimura, pp. 178-179, 1989. In addition, the described deterioration occurs in the Continuous Czochralski (CCZ) method in which a crystal of silicon is manufactured while a silicon material is continuously being supplied to a crucible. In these methods, the operation time of the quartz crucible becomes longer, so that the inner surface of the quartz crucible deteriorates, resulting in the described generation of dislocations in crystals of silicon. Therefore, for example, in the multi-pulling method, production of a crystal of silicon becomes impossible to continue, and in the CCZ method, production of a crystal of silicon must be stopped part way.
U.S. Pat. No. 5,357,898 to Kirosawa et al., discloses a method of producing a crystal by bringing an electrode in contact with a melt, applying electric current through the melt between the electrode and a metallic container and controlling the current so as to keep it at nearly zero. The intent of the disclosed method is to minimize the electrochemical reaction between the metallic crucible and the melt. The metal container is made mainly of platinum, or a platinum alloy.
U.S. Pat. No. 4,330,359 to Shlichta discloses an electromigration process for the purification of molten silicon during crystal growth. A direct current (DC) power source is provided for applying a current to the silicon melt. The emerging silicon crystal forms the negative electrode while the crucible is preferred as the positive electrode. Applied DC voltages could be on the order of 1 to 10 volts at a current of over 100 amps. At these voltages and currents, resistance heating of the crystal would be expected to occur. It would not be possible to apply this disclosed process to a quartz crucible, commonly used in Czochralski crystal growth, in the electrical circuit. The electrical resistance of the circuit through the crucible would not permit current levels in excess of 100 Amps with a potential of only 1 to 10 Volts. The present inventors estimate that voltages in excess of 80,000 Volts DC would be necessary to achieve current levels over 100 Amps.
Accordingly, there exists a need in the art for a method of manufacturing a crystal of silicon that can reduce the possibility of causing deterioration of the inner surface of a quartz crucible or can restore the deteriorated inner surface of the crucible, thereby making it possible to manufacture a crystal of silicon having a large diameter while reducing the generation of dislocations. Further, in relation to the multi-pulling method, the need exists for a method to manufacture a larger number of crystals of silicon from a single quartz crucible, and in relation to the CCZ method, it has been demanded to operate over a longer period of time using a single quartz crucible, thereby making it possible to manufacture more and/or longer crystals.